One of the most significant changes in light weight coated (LWC) paper production is the use of the short-dwell coater. The short-dwell coater has enabled the paper maker to improve productivity while maintaining coated paper quality. The term "short-dwell" refers to the relatively short period of time that the coating is in contact with a web of paper material before the excess is metered off by a trailing doctor blade. As shown in FIG. 1, prior art short-dwell coaters consist of a captive pond 21 just prior to the doctor blade 23. The pond is approximately 5 cm in length and is slightly pressurized to promote adhesion of the coating to the paper web 25. The excess coating supplied to the sheet creates a backflow of coating 27. This coating backflow excludes to some extent the boundary layer of air entering with the sheet and eliminates skip coating. The excess coating is channeled over an overflow baffle 29 and collected in a return pan before returning to tanks to be screened.
While short-dwell coaters are extensively used in coating paper webs, such coators suffer from a major problem. The flow in the coating chamber of the pond upstream of the doctor blade contains recirculating eddies or vortices which can result in coat-weight nonuniformities and wet streaks or striations in several ways. For example, these eddies can become unstable due to centrifugal forces and result in the generation of unsteady flow and rapidly fluctuating vortices, which deteriorate the coating uniformity and its quality. Also, the vortices tend to entrap small air bubbles which result in the buildup of relatively large air inclusions in the coating liquid which tend to accumulate in the core region of the eddies. Vortex fluctuations tend to force these air inclusions into the blade gap. This adversely affects the coating quality. Usually, the presence of air inclusions results in regions of lower coat weight which are 2-4 cm wide and about 10-100 cm long, known in the industry as "wet streaks". These problems are discussed in an article "Principles of Hydrodynamic Instability: Application in Coating Systems", C. K. Aidun, Tappi Journal, Vol. 74, No. 3, March, 1991.
It would be desirable to provide a coating device which has the coating advantages of a short-dwell coater, but which did not have the problems associated with recirculating eddies or vortices and the entrapment of air pockets or air bubbles in the core of the vortices.
Accordingly, it is a principal object of the present invention to provide a vortex free short-dwell coating device.
These and other objects will become more apparent from the following description and the appended claims.